Abstract:

Whilst the use of expansion joints is common practice in bridge construction,
modular bridge expansion joints are designed to accommodate large longitudinal
expansion and contraction movements of bridge superstructures. In addition to
supporting wheel loads, a properly designed modular joint will prevent rain water
and road debris from entering into the underlying superstructure and substructure.
Modular bridge expansion joints (MBEJs) are widely used throughout the world
for the provision of controlled pavement continuity during seismic, thermal
expansion, contraction and long-term creep and shrinkage movements of bridge
superstructures and are considered to be the most modern design of waterproof
bridge expansion joint currently available. Modular bridge expansion joints are
subjected to more load cycles than other superstructure elements, but the load
types, magnitudes and fatigue-stress ranges that are applied to these joints are not
well defined. MBEJs are generally described as single or multiple support bar
designs. In the single support bar design, the support bar (beam parallel to the
direction of traffic or notionally parallel in the case of the swivel joist variant)
supports all the centre beams (beams transverse to the direction of traffic) using
individual sliding yoke connections (for the swivel joist variant, the yoke
connection is characterised as a one-sided stirrup and swivels rather than slides).
In the multiple support bar design, multiple support bars individually support each
centre beam using a welded connection.
Environmental noise complaints from home owners near bridges with modular
expansion joints led to an engineering investigation into the noise production
mechanism. It was generally known that an environmental noise nuisance
occurred as motor vehicle wheels passed over the joint but the mechanism for the
generation of the noise nuisance has only recently been described. Observation
suggested that the noise generation mechanism involved possibly both parts of the
bridge structure and the joint itself as it was unlikely that there was sufficient
acoustic power in the simple tyre impact to explain the persistence of the noise in
the surrounding environment.
Engineering measurements were undertaken at two bridges and subsequent
analysis led to the understanding that dominant frequency components in the
sound pressure field inside the void below the joint were due to excitation of
structural modes of the joint and/or acoustic modes of the void. This initial
acoustic investigation was subsequently overtaken by observations of fatigue
induced cracking in centre beams and the welded support bar connection. A
literature search revealed little to describe the structural dynamics behaviour of
MBEJs but showed that there was an accepted belief amongst academic
researchers dating from around 1973 that the loading was dynamic. In spite of
this knowledge, some Codes-of-Practice and designers still use a static or quasistatic
design with little consideration of the dynamic behaviour, either in the
analysis or the detailing. In an almost universal approach to the design of modular
bridge expansion joints, the various national bridge design codes do not envisage
that the embedded joint may be lightly damped and could vibrate as a result of
traffic excitation. These codes only consider an amplification of the static load to
cover sub-optimal installation impact, poor road approach and the dynamic
component of load. The codes do not consider the possibility of free vibration
after the passage of a vehicle axle.
Codes also ignore the possibilities of vibration transmission and response
reinforcement through either following axles or loading of subsequent
components by a single axle. What the codes normally consider is that any
dynamic loading of the expansion joint is most likely to result from a sudden
impact of the type produced by a moving vehicle ‘dropping’ onto the joint due to
a difference in height between the expansion joint and the approach pavement.
In climates where snow ploughs are required for winter maintenance, the
expansion joint is always installed below the surrounding pavement to prevent
possible damage from snow plough blades. In some European states (viz.
Germany), all bridge expansion joints are installed some 3-5mm below the
surrounding pavement to allow for possible wear of the asphaltic concrete. In
other cases, height mismatches may occur due to sub-optimal installation.
However, in the case of dynamic design, there are some major exceptions with
Standards Australia (2004) noting that for modular deck joints “…the dynamic
load allowance shall be determined from specialist studies, taking account of the
dynamic characteristics of the joint…” It is understood that the work reported in
Appendices B-E was instrumental in the Standards Australia committee decisions.
Whilst this Code recognizes the dynamic behavior of MBEJs, there is no guidance
given to the designer on the interpretation of the specialist study data. AASHTO
(2004), Austrian Guideline RVS 15.45 (1999) and German Specification TL/TPFÜ
92 (1992) are major advancements as infinite fatigue cycles are now specified
and braking forces considered but there is an incomplete recognition of the
possibility of reinforcement due to in-phase (or notionally in-phase) excitation or
the coupled centre beam resonance phenomenon described in Chapter 3.
This thesis investigates the mechanism for noise generation and propagation
through the use of structural dynamics to explain both the noise generation and the
significant occurrence of fatigue failures world-wide. The successful fatigue
proofing of an operational modular joint is reported together with the introduction
of an elliptical loading model to more fully explain the observed fatigue failure
modes in the multiple support bar design.